diff --git a/programs/develop/kosjs/Tupfile.lua b/programs/develop/kosjs/Tupfile.lua
index eb80c57b14..34ca77fe9e 100755
--- a/programs/develop/kosjs/Tupfile.lua
+++ b/programs/develop/kosjs/Tupfile.lua
@@ -3,6 +3,7 @@ HELPERDIR = (tup.getconfig("HELPERDIR") == "") and "../.." or tup.getconfig("HEL
 tup.include(HELPERDIR .. "/use_gcc.lua")
 tup.include(HELPERDIR .. "/use_newlib.lua")
 
+CFLAGS = CFLAGS .. " -std=c99 -Wall -Wextra"
 INCLUDES = INCLUDES .. " -I ."
 
 compile_gcc{"libmujs/utftype.c", "libmujs/jsproperty.c", "libmujs/jsobject.c", "libmujs/jsdtoa.c", "libmujs/jsbuiltin.c", "libmujs/jsvalue.c", "libmujs/jsnumber.c", "libmujs/jsparse.c", "libmujs/jsstate.c", "libmujs/jsgc.c", "libmujs/jsrepr.c", "libmujs/pp.c", "libmujs/utf.c", "libmujs/jsfunction.c", "libmujs/jsdump.c", "libmujs/regexp.c", "libmujs/jsstring.c", "libmujs/jsarray.c", "libmujs/jsrun.c", "libmujs/jsdate.c", "libmujs/jscompile.c", "libmujs/jslex.c", "libmujs/jsboolean.c", "libmujs/jserror.c", "libmujs/jsintern.c", "libmujs/jsmath.c", "libmujs/jsregexp.c", "libmujs/json.c"}